1. Technical Field. This invention relates to boilers for generating vapor from liquid. More particularly, this invention relates to a boiler in which the liquid to be vaporized is fed by capillary action.
2. Background: Boilers are used to convert liquid to vapor in applications in which vapor is necessary, or preferable, to liquid. All boilers serve to add heat to an inflow of liquid in order to vaporize the liquid and create an outflow of vapor. The pressure of vapor generated by a boiler cannot exceed the pressure of the supplied liquid. Therefore, to supply vapor under pressure, an inflow of liquid to the boiler must be supplied under at least as much pressure as is desired for the vapor.
Liquid inflow to large industrial boilers is commonly supplied by a mechanical or jet-ejector feed pump that draws liquid from a reservoir at atmospheric pressure. These feed pumps deliver liquid to the boiler at a pressure at least as great as that desired for the vapor. A throttle valve is typically used to control the flow of vapor from the boiler, and the pressure of the vapor exiting the boiler is a function of the position of the throttle valve. Feed pumps maintain a constant liquid level in a boiler. They do this over a reasonable range of vapor flow and pressure as determined, for example, by a throttle valve position. The liquid flow produced by mechanical or jet ejector feed pumps on boilers is therefore servo controlled to maintain a constant liquid level in the boiler. It is not practical to scale down this kind of system for producing the low vapor flow requirements of devices such as domestic stoves, camp stoves, or mantle lamps.
Camping stoves and other portable burners require the production of gaseous fuel to be mixed with air and combusted. Fuels, such as propane and butane, which are gasses at atmospheric temperature and pressure, are liquids under pressure and occupy smaller volumes for economical storage and transport. This necessitates the use of pressurized containers, with the attendant explosion hazards. Similar hazards exist when the liquid fuel is supplied to a boiler from a reservoir pressurized with gas or air, as in the case of gasoline stoves and mantle lamps.
The boiler of propane and butane stoves is the reservoir or storage tank itself, in which the gasses are liquified under pressure. When vapor is drawn from the reservoir, the reservoir acts as a boiler, and draws the required heat of vaporization from ambient air outside the tank. These types of stoves have many disadvantages. For example, the vapor pressure depends upon ambient temperature, the vapor pressure is generally higher than that needed for satisfactory combustion in a burner and, as previously mentioned, the fuel and vapor are at hazardous pressures. While butane fuel has an advantageous lower vapor pressure than propane, stoves using butane have difficulty producing sufficient vapor pressure at low ambient temperatures. The pressure of propane does not fade at low ambient temperatures. But the vapor pressure of propane nonetheless varies with the tank or ambient temperature and the pressure is inconveniently high. A needle valve can control propane vapor at tank pressure to regulate the heat output of a burner. But burner control by a needle valve tends to be delicate and sensitive to ambient temperature. Alternatively, a pressure regulator can be used to generate a constant and less hazardous pressure of propane that is independent of tank temperature. These are reasons why pressure regulators are commonly used in cookout grilles, recreational vehicles, boats, and domestic propane installations. Unfortunately, regulators are seldom practical for applications at the scale of camp stoves.
It is, therefore, an object of this invention to provide vapor at pressures higher than the pressure of the liquid from which the vapor is created without the use of feed pumps.
It is another object of the present invention to provide vapor at pressures higher than the pressure of the liquid from which the vapor is created without pressurizing the liquid.
It is a further object of the present invention to provide vapor at an approximately constant pressure that is not strongly dependent upon ambient temperature or upon mass flow of the vapor.
Another object of this invention is to provide vapor at a steady flow rate.
Yet another object of the present invention to provide an economical portable stove fueled by unpressurized liquid fuel without the use of feed pumps.